The invention concerns a process for reducing the benzene content of petrol (gasoline) fractions. In the context of the present invention, these petrol fractions are preferably and generally mixtures of light reformate and a straight run C.sub.5 -C.sub.6 cut. Said process combines hydrogenation of a feed such as the light reformate and optional hydrogenation of a C.sub.5 -C.sub.6 cut with isomerisation of the effluent from hydrogenation and optional isomerisation of said cut. The process is characterised in that the isomerisation reaction is carried out using a particular catalyst which contains chlorine and at least one metal from group VIII deposited on a support constituted by a mixture of specific proportions of eta alumina and gamma alumina.
Environmental problems have led to a joint reduction in the lead content and the benzene content in petrol fractions, preferably without reducing the octane number. Catalytic reforming under very severe conditions and isomerisation of normal C.sub.5 -C.sub.6 paraffins with a low octane number are currently the most frequently used processes for producing high octane numbers without adding lead. However, catalytic reforming produces large quantities of benzene with a high octane number. Thus it is necessary to develop new processes which can reduce the benzene content in a petrol while complying with octane number specifications.
The combination of catalytic reforming and isomerisation, consisting in separating the C.sub.5 -C.sub.6 fraction from the reformate, isomerising it and introducing it directly into petrol fractions to improve the octane number, are well known: this is, for example, described in U.S. Pat. Nos. 4,457,832, 4,181,599 and 3,761,392. Isomerisation of the C.sub.5 -C.sub.6 cut from a straight run crude is also well known. The octane number of the cut is considerably improved. The benzene content of the reformate can be reduced in different ways, such as modifying the naphtha cut point between reforming and isomerisation or separating the reformate into two fractions: a heavy fraction (heavy reformate) and a light fraction (light reformate), all the benzene being concentrated in said light fraction. The light fraction is then sent to a hydrogenation unit which transforms the benzene into cyclic hydrocarbons which are then ring opened in an isomerisation unit working under severe conditions. The normal paraffins formed are isomerised using a conventional isomerisation process (U.S. Pat. No. 5,003,118).
European patent application EP-A-0 552 070 concerns a process comprising hydrogenation of a feed with the following composition by weight: 40% to 80% of paraffins, 0.5% to 7% of cyclic hydrocarbons and 6% to 45% of aromatics. The process is characterised in that the maximum distillation temperature is between 70.degree. C. and 90.degree. C. and in that the hydrogenation effluent is isomerised in an isomerisation step, said charge and/or said effluent being mixed with a C.sub.5 -C.sub.6 cut.